Risk Evaluation of Construction Workers' Exposure to Silica Dust and the Possible Lung Function Impairments.

Background
Aerosols generated during construction activities are an integral part of building operations. Considering the nature of materials used in construction activities, respirable dust contains crystalline silica and particulates not otherwise specified (PNOS). Due to lack of data regarding the occupational health status of Iranian construction workers, the objective of this study was to evaluate occupational exposure to silica and to examine their respiratory health status.


Materials and Methods
In this cross sectional study, 85 construction workers and 40 controls (without active exposure to construction dust) were studied. The workers' exposure to PNOS and silica aerosols was monitored by the NIOSH method No.0600 and a new Fourier transform infrared spectroscopy (FTIR)-based method, respectively. All subjects were also monitored for lung function parameters, such as forced expiratory volume/forced vital capacity (FEV1/FVC), peak expiratory flow rate (PEFR), forced expiratory flow (FEF25-75), FVC, and FEV1.


Results
The mean exposure of workers to respirable PNOS and silica was 9.8 (0.35) and 0.13 (0.019) mg/m3, respectively. The groups of construction workers showed significant differences in exposure to PNOS (P< 0.001) and silica (P= 0.007). The mean pulmonary function parameters, including FEV1% and FVC%, were significantly lower among construction workers, compared to the control group (P< 0.001 and P= 0.009, respectively). The pulmonary status of 51.8% of construction workers showed moderate restriction, while 4.70% exhibited obstruction.


Conclusion
Considering the construction workers' excessive exposure to PNOS and silica, besides depressed lung function parameters, they can be classified as a high-risk group for respiratory diseases.


INTRODUCTION
Construction activities reportedly generate dusts, which can be a risk factor for workers' respiratory dysfunctions (1)(2)(3)(4). Lung function impairment is the most common respiratory problem among workers exposed to dusts (5). Construction workers are especially exposed to high concentrations of dusts in closed spaces and breathe high levels of crystalline silica (6-9). The International Agency for Research on Cancer (IARC), based on sufficient evidence of carcinogenicity, has classified crystalline silica as a group I carcinogen and a definite human carcinogen (9,10). The American Conference of Governmental TANAFFOS Industrial Hygienist (ACGIH) has also classified silica in group A2 as a probable carcinogen (11). Due to the carcinogenicity of crystalline silica, ACGIH reduced the threshold limit value (TLV) of crystalline silica from 0.1 mg/m 3 in 1986 to 0.025 mg/m3 in 2006 (12).
Silicosis is recognized as a restrictive pulmonary disease. It has been described as the most prevalent respiratory disease since 1968 due to silica dust exposure and is now considered a global problem (13)(14)(15). Since 1995, known as Elimination of Silicosis in the Americas [16].
Considering the nature of materials used in the construction industry, dust may contain significant amounts of crystalline silica (16,17). There is substantial epidemiological evidence in relation to occupational exposure to respirable general dusts, which contain less than 1% silica and are classified as particulate not otherwise specified (PNOS), as well as respirable crystalline silica, associated with the development of various diseases, such as silicosis, lung cancer, tuberculosis, and pulmonary obstructive disease (15,(18)(19)(20)(21) (33). Fourier transform infrared spectroscopy (FTIR), due to the application of a Fourier algorithm, is more accurate for the recognition of crystalline silica molecular fingerprint in comparison with infrared spectrophotometry (34).
Construction workers reportedly experience greater exposure to respirable crystalline silica and PNOS compared to the occupational exposure limit, and several authorities have recommended further research for better monitoring and control of construction workers (35,36).
According to the International Labor Organization (ILO), silicosis, as a preventable but incurable disease, requires awareness of the quality and quantity of respirable crystalline silica for devising proper control measures (37,38). Considering the high occupational exposure of construction workers to airborne dusts and absence of relevant studies in Iran, the aim of this study was to evaluate workers' exposure to respirable PNOS and crystalline silica and to examine their respiratory lung function status.

MATERIALS AND METHODS
Eighty-five workers from a major construction company, along with 40 control workers without active exposure to respirable crystalline silica and PNOS were randomly selected, according to the statistical calculations related to pulmonary function from a previous study, monitoring a group of construction workers (26). All workers were from the same socioeconomic class, nonsmokers, and healthy with at least 1 year of work history, without any respiratory diseases or prescription drug use. Iranian construction workers were from 5 occupational task groups, including supervisors of construction activities, cleaning crew, and cement, masonry, batching, and concrete workers. The control workers, without active exposure to dust, were recruited from the security personnel of the same site.
In this study, the workers' personal exposure to respirable PNOS was determined according to the NIOSH method No. 0600 (33). In this method, personal monitoring was performed with an SKC personal sampling pump The prepared tablets were scanned using an FTIR spectrometer (model WQF-510A) at the wavelength range of 400-4000 cm -1 . Crystalline silica in the prepared tablets was determined in the range of 710-825 cm -1 . In order to examine the accuracy of FTIR analysis, 13 bulk samples were analyzed by the FTIR-based method of this study, as well as a reference method combining X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses (39). For this purpose, an XRD device (PW1800, Philips Co.) was used for the qualitative detection of crystalline silica, and an XRF device (PW1480, Philips Co.) was applied for quantitative detection.
Pulmonary function tests were performed using a spirometer (model 3000, Bionet Cardio Touch) in the exposed and control groups. The characteristics (height, weight, and age) of the exposed and control groups were recorded. The subjects were asked to stand comfortably in front of the spirometer and then inhale and exhale. The

RESULTS
The mean age and work experience were 32.32 and 10.8 years in the exposed group and 32.09 and 9.78 years in the control group, respectively. There was no significant difference in terms of age and work experience between the exposed and control groups (P> 0.05). Agreement of 2 datasets from 1 set of silica samples (13 samples) was analyzed by 2 different methods (FTIR and combination of XRD and XRF) and examined by the Bland-Altman plot.
As the differences were within ±0.2 SD from the average of differences, the agreement of 2 datasets was established (ICC, 0.993; P< 0.001). Table 1 Table 2).
In addition, significant differences were found between the normal and abnormal pulmonary status of the construction and control groups (P< 0.001). The pulmonary status was normal in 43.5% of subjects from the construction groups and 87.5% of subjects from the control group. Also, more than half of the construction workers (51.8%) were diagnosed with moderate restriction, while 4.70% showed the obstructive status ( Figure 1).
According to the regression analysis, exposure to respirable PNOS and cumulative exposure to PNOS had a significant negative correlation with respiratory parameters, FVC and FEV1, respectively (Figures 2-5). A significant negative correlation was also observed between cumulative exposure to respirable crystalline silica dust and the respiratory parameter, FVC ( Figure 6).

DISCUSSION
In this study, there were no significant demographic differences between the construction and control groups.